Published Oct 6, 2017 at 7:19 PM

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The Santa Clara Valley Transportation Authority and BART are planning to collaborate, starting this month, on a peer review of single-bore and twin-bore tunnel methodologies for a five-mile span of tunnel planned for the Silicon Valley extension.

The peer review will be conducted by transit agencies currently operating railway subway systems. VTA spokeswoman Brandi Childress said the transit system is also looking at particular agencies to coordinate within the coming weeks.

There are currently two proposals under consideration: one, a 45-foot diameter single-bore tunnel that will have the two tracks and the stations within it and the second is a twin-bore tunnel that has two 20-foot diameter tunnels with stations and a wide-center platform.

The Bart Phase II Silicon Valley extension is a long-term plan to extend BART from its original terminus point in Fremont into Silicon Valley, which was agreed upon by the VTA and BART in 2001.

The VTA said in a statement that the Phase II part of the project is currently in the state and federal environmental analysis process and is on track to receive the federal go-ahead in March 2018. Phase II, which has plans for three more stations south of the Berryessa station, is expected to be complete with an extension into downtown San Jose and Santa Clara by 2026.

The Warm Springs/South Fremont extension has been operational since March after years of delays, but it is not part of the BART Silicon Valley extension. Childress said one of the important factors in the Silicon Valley extension is eventually turning the Warm Springs/South Fremont station into a pass-through station as opposed to an end-of-line station.

The Milpitas station and the Berryessa station are the next two scheduled to open in 2018.